Heavy alternating-current measurement



Oct. 4, 1938. w LORD 2,132,267

HEAVY ALTERNATING CURRENT MEASUREMENT Filed July 51, 1957 lmverwtor:Harold W. Lord,

b A/ MZa M Patented Oct. 4, 1938 UNITED STATES PATENT OFFICE.

HEAVY ALTEBNATING-CUBRENT MEASURE- MENT Harold w. Lord, Schenectady, N.1., assignor to General Electric Company,

New rel-k a corporation of Application July 31, 1937,'Serial No. 156,746

My invention relates to the measurement of heavy alternating currentsand its object is to avoid the, use oi the expensive and massive curbetween the circuit to be metered and an instrument circuit. Theinstrument circuit includes an inductance in order that the currenttherein will have a wave shape which corresponds to the wave shape oithe current to be measured. In some no cases, I may combine the pick-upcoil and waveshape-correcting reactance into a single device. Thefeatures oi my invention which are believed to be novel and patentablewill be pointed out in the claims appended hereto. For a betterunderstanding oi my invention, reierence is made in the iollowingdescription to the accompanying drawing in which Iflg. 1 represents thecircuit connections according to my invention ior obtaining anoscillograph record of the currentso flowing in a resistance weldingcircuit; Fig. 2

represents a circuit ior thesame purpose wherein the pick-up coil andinductance are combined; and Fig. 3 shows a perspective view oi aportion oi a resistance welder with mypick-up .coil inmeasuring'positionthereon'. 1

Referring now to the drawing ior a, detail description, in Figs. 1 and2, ll represents a transformer having a primary II and secondary i2connected between an alternating-current source oi supply It and theelectrodes ll oi a resistance welder. The transformer II is' designed toproduce a very large low-voltage secondary current suitable iorresistance welding purposes. The

' welding current-through the electrodes may,

ior enmple, reach momentarily values up to 200.000 amperes or more.

It is impracticable to use theconventional design oi current-measurementtransiormer or hunt in the welding circuit where such large currentmagnitudes are involved. It is-.desirable,

however.toobtainaproportional instrumentcurrentoi the same wave shapeaathatoi the welding current. In accordance withmy invention,

thisisaccomplishedbyplacingaplck-upcoil ll .inthe throat oi the welderwhere itis inductively related to the welding circuit, and connecting aninstrument. such as an oscillograph element I6,

in circuit with this pick-up coil.' Thus'in Figs. 1

and 2 the welding circuit comprises a single turn loop with which thepick-up coil is in'loose inductive relation. The voltage e induced inthe pick-up coil is a function of the mutual inductance 3n between suchcoil and welding circuit and the rate oi change 01' current 10 a: in thewelding circuit, thus ii I 5 m It is necessary to include a reactor H inthe measuring circuit in order that the current therein shall deflectthe oscillograph element It in accordance with the wave shape oi thewelding cur- 2o rent. The instrument circuit should be oi low resistanceas compared to its inductance. Neglecting the eiiect oi resistance thecurrent 11 in the oscillograph circuit'is then related to the voltage einduced in the pick-up coil in accord- 25 ance with the expression whereL is the inductance of the reactor l1. and 30 represents the rate oichange oi current in the osciilosraph circuit. 35

We may combine the above two equations by eliminating'the common iactore, thus i' "'4: qt

mv and L are circuit constants and hence I v =lik where k is a constantdepending upon the circuit constants. Thus,-the current 11 flowing in i5the oscillograph is proportional to the weldins current Landthesecurrents have substantially the same wave shape and phase relation. Theproportionality between i and 11 can be determined by calibration.'However, since the caliso bration depends upon the mutual inductancebetweenthe pick-upcoil and welding circuit, the pick-up coil should beimmovably placed in the throat oi the welder before calibration and notmoved thereafter without recalibration. The

order of tions.

transformation ratio between i and i1 is of the 1 and above.

A measurement of the oscillograph deflection for a known welding currentprovides a reasonably accurate overall calibration. The welding currentfor low heats with the electrodes shortcircuited may be fairlyaccurately determined by measuring the steady state current in theprimary of transformer I and multiplying this value by the turn ratio ofthis transformer. The pick-up coil may be provided with different taps,such as indicated at I8, Fig. l, for convenience in varying thecalibration tosuit the different operating currents for differentwelding condi- In order to give a practicable example but not by way oflimiting the invention, it has been found that the pick-up coil may haveeight hundred turns of No. 25 enameled wire wound on. a two-inchdiameter tube three inches long and may have taps at two hundred andfour hundred turns. This coil Weighs about ten ounces. The reactor usedat I! with this pick-up coil is an iron core reactor with a small airgap rated at one henry with .3 ampere direct current through its windingand weigh about one pound. The placing of the pick-up coil on aresistance welding machine is represented in Fig. 3.

As shown in Fig. 2, the pick-up coil l and reactor of Fig. 1 may becombined by using a reactor I9 as both pick-up coil and reactor. Suchpick-up reactor must be placed relative to the welding circuit so thatthe current flowing in the K oscillograph vibrator circuitis justsuficient for ment circuit current will, of course, vary with a gooddeflection. Too strong a field in the pickup reactor will saturate theiron thereof and cause distortion in the recording. i

If the pick-up reactor is used and the welding current varies over aconsiderable range, it will be'advisable to mount the pick-up reactor sothat it may be turned through an arc of ninety degrees by means such asindicated in Fig. 2 at 20 and thus vary its inductive relation withrespect to the welding circuit for difierent ranges of welding current.The calibration of the instruthe position of the pick-up reactor and,after being calibrated, different rotary positions of the reactor may bemarked with the proper calibration constants. I

The instrument at l6 instead of being of the recording type mayobviously be of the indicatby other means.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a primary circuit in which alternating current offrom 50,000 to 200,000 amperes may flow, apparatus for measuring suchcurrents in said circuit comprising, a'pick-up coil loosely inductivelyrelated to said circuit, an instrument supplied by said pick-up coil,and a reactorv having an iron core containing an air gap connected inseries with said instrument and pick-up coil, said reactor serving tocause the wave shape of the current induced in said instrument circuitthrough said pick-up coil to be similar to the wave shape of the currentin the primary circuit, the ratio between the currents in the primaryand instrument circuits being not less than 2. In combination, analternating-current welding machine having electrodes and electricalsupply conductors leading to'the electrodes and forming a single-turnconductor loop when current flows through said conductors and betweensaid electrodes in a welding operation,

means for obtaining a record of the current flow in said conductorscomprising, a coil inserted in such loop so as to form therewith aloosely cou-" pled mutual inductance coupling, an electrical recordingdevice of the oscillograph type con-' nected to be energized from saidcoil, and a re-, actance included in the energizing circuit of saidrecording device for causing the wave shape of the current inducedtherein through said mutual inductance coupling to conform to the waveshape of the current flow in the welder supply conductors.

3. In combination, a heavy-current alternating-current circuit forming asingle-turn loop,

means for measuring the current flow in said circuit comprising, a coilplaced within said loop so as to have a voltage induced therein whencurrentlflows in said circuit, which voltage is proportional to the rateof change of current'in measuring circuit containing sufllcientinductance to cause the wave shape of the current in the instrumentcircuit to 'be siinilar to the wave shape of the current in theheavy-current circuit.

4. In combination, a heavy-current alternating-current circuit in whichcurrents of the or- AROLD w. Loan.

